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All IPCC definitions taken from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Annex I, Glossary, pp. 941-954. Cambridge University Press.

Posted on 7 February 2011 by dana1981

Dr. Roy Spencer, like Dr. Richard Lindzen (the subject of a fewrecent articles), is one of very few climate scientists who remain unconvinced that most of the the recent global warming has been caused by humans (anthropogenic). Dr. Spencer has grown frustrated with the fact that most of his climate scientist colleagues conduct research under the premise that the recent warming is anthropogenic, and in an article on his blog, has thrown down the gauntlet:

"Show me one peer-reviewed paper that has ruled out natural, internal climate cycles as the cause of most of the recent warming in the thermometer record."

This challenge is problematic for a few reasons. Firstly, the fact that research has not ruled out a hypothesis does not mean the hypothesis necessarily has any validity. For example, there have been no peer-reviewed papers ruling out leprechauns as the cause of most of the recent global warming, either. But perhaps more importantly, our understanding that humans are causing global warming is not based on just one scientific study, but rather a very wide range of scientific evidence.

For example, scientists have measured the amount of heat being re-directed back towards the Earth's surface due to the increased greenhouse effect. Quantifying the amount of global warming that this will cause simply involves multiplying the increased downward energy by the climate sensitivity. As the name suggests, climate sensitivity is a measure of how sensitive the climate is to this build-up in heat - how much the planet will warm in response to an increase in the amount of greenhouse gases in the atmosphere.

Therefore, in order to argue that humans are not the driving force behind the current global warming, skeptics like Spencer and Lindzen require that the climate sensitivity to increasing greenhouse gases is low. The problem with this position is that there are many lines of evidence that the planet will warm between 2 and 4.5 degrees Celsius (°C) if the amount of carbon dioxide (CO2) in the atmosphere doubles.

For example, some scientists have studied the climate response to recent large volcanic eruptions, which can have a measurable impact on global temperatures. Other studies have examined how the global temperature has changed in response to changes in solar activity. Some other research has compared CO2 and global temperature changes over the past thousand years, and tens of thousands of years, and hundreds of thousands of years, and even millions of years ago. We can even compare how the temperature has changed over the past century to human-caused atmospheric CO2 changes. In every case we arrive at this same climate sensitivity range of 2 to 4.5°C, and the most likely value is 3°C for a doubling of atmospheric CO2.

If we take the lower end of this range, even a 2°C climate sensitivity would mean that humans have been responsible for more than half of the global warming over the past century. So in order for Spencer and Lindzen to be right, all of these different lines of evidence which are in agreement with the likely range of climate sensitivity would all have to be somehow wrong, and all biased high. Not an impossibility, but certainly not a likely scenario, either.

There are also many "fingerprints" of human-caused global warming. For example, as the Earth's surface and lower atmosphere have been warming, the upper atmosphere has been cooling. There are not many mechanisms which can explain these observations, but they are precisely what we would expect to see from human-caused global warming. As the concentration of greenhouse gases in the lower atmosphere increases, they effectively trap more and more heat in this lower layer, causing it to warm and causing the layers above to cool. Another human "fingerprint" is the higher rate of warming at night than during the day. This is because at night, when the surface is cooler and not being bombarded by solar energy, the increased amount of greenhouse gases are able to make more of a difference in the surface temperature.

Dr. Spencer has proposed an alternative to the anthropogenic global warming theory. He suggests that some unknown mechanism has caused global cloud cover to decrease over the past century. Low-level clouds cause a cooling effect by reflecting sunlight, so if these types of clouds become less prevalent, it can cause the surface to warm. However, this hypothesis cannot explain the "fingerprints" describe above. A decrease in cloud cover would not cause the upper atmosphere to cool. Nor would it cause nights to warm faster than days - quite the opposite. Cloud reflectivity only plays a significant role during the day when being bombarded by sunlight.

Dr. Spencer also suggested in his blog post that the "null hypothesis" should be that global warming is caused by natural factors. A null hypothesis is basically the default assumption which a scientific study sets out to disprove. It's true that until recently, global warming (and cooling) has been caused by natural factors. However, even natural climate changes must have a physical mechanism causing them. Scientists have investigated these natural mechanisms (the Sun,volcanoes, the Earth's orbital cycles, etc.), and they simply cannot explain the global warming over the past century. Spencer's new hypothesis - that some unknown mechanism is causing cloud cover to change, which in turn is driving global temperatures - is a new idea with very little supporting evidence. Conversely, our understanding that human greenhouse gas emissions are driving global temperatures has a proverbial mountain of supporting evidence.

Skeptics like Spencer and Lindzen believe that the default assumption should be one which requires that a very large body of scientific evidence is wrong. The only alternative hypothesis they have put forth cannot explain the many empirically-observed "fingerprints" which are consistent with human-caused global warming. Although Spencer's unspecified "natural internal cycle" hypothesis has not been explicitly disproved, there is a very low likelihood that it is correct. For this reason, we should operate under the assumption that humans are causing dangerous global warming - an assumption which is supported by a very large body of evidence - until the skeptics can provide solid reason to believe that this scientific theory is wrong.

A cogent argument that Dr Spencer is wrong. It is all very well to put forward a null hypothesis and claim that it offers an alternative explanation but such claims have to be sustained by empirical evidence or, at the very least, be consistent with and explain such evidence. Dr Spence does neither. Anthropogenic global warming does both.

The ALO* explanation, which shows that it's actually cooling, is more credible than that proposed by Dr Spencer. I found this terrific explanation on Tamino's blog at Open Mind, scroll down to see the upcoming WUWT post by Ned:

7 chemware: there is a suggested cause of a change in cloudcover. If cloud cover's radiative forcing is nonlinear in temperature (say), then simply by redistributing heat around the world you get a non-zero change in cloud cover.

e.g. cloud forcing might be related to temperature by C = T2. In which case if you had 2 places at T=10, you start with C=200.

But if you move 1 up by 1 and the other down by 1, such that new temperatures are 9 and 11 then you still have the same mean temperature but C=202.

This highlights the principle and I think it makes sense. But the evidence for climate sensitivity of order 3 C to CO2 doubling is very strong. IMO it's possible that Spencer is right, and it's about the only non-AGW theory that actually makes sense as an explanation for recent warming... and there should be more work done on it. Spencer has demonstrated that a strong negative cloud feedback might exist, but multiple lines of evidence suggest that it doesn't which makes me feel pretty confident that we don't have one. And without a strong negative cloud feedback or a massive screw up in the water vapour one then most global warming since the 70s is human caused.

Good effort. I think you refer 'natural' as 'that which has not been modified by tools or been a subject to the usage of tools or has used tools' and 'internal' as 'referring to a happenstance inside of a specified system, exluding any supernatural influence' or something like that, I reserve the right formulate these definitions better, as leprechauns are thought to exist by some.

(1) The leprechauns comparison is unfair. We know climate change can be natural, but we don't have any evidence leprechauns exist. The plausibility of the first is thus several orders of magnitude greater than the plausibility of the second.

(2) Your discussion of onus is a bit confused. Spencer doesn't suggest his cloud theory should be the null hypothesis, he's suggesting one or another natural process should be the null hypothesis. (Which is rather different). And in the absence of studies showing global warming is anthropogenic, the onus is clearly on showing it is. What's happened is that the null hypothesis has shifted due to the weight of evidence in favour of AGW.

Surely there are numerous peer-reviewed papers (e.g. Hansen) that model the temperature based on the various contributing factors and conclude that observations only match theory if one includes CO2 increases. Combined with other papers that report the increases in CO2 cannot be from volcanoes or termites and that it is man, then we have a reply to Spencer's challenge ruling out natural cycles.

The fundamental problem with Spencer's hypothesis is that it is unfalsifiable. As there is no specified mechanism or model that can be used to predict what observations are and are not consistent with the hypothesis, it is impossible to disprove. Karl Popper (very influential figure in the philosophy of science) wrote that the thing that distinguishes science from pseudo-science (e.g. astrology) is that there is always at least the possibility that a scientific hypothesis can be falsified. This means the hypothesis needs to make prohibitions on what can happen, the more prohibitions it makes, the better the theory. Clearly Spencer's hypothesis of "natural cycles" is not a scientific hypothesis, at least until he specifies the mechanism sufficiently that something is ruled out by it.

Essentially, you can't prove any hypothesis by experiment or observation, all you can do is disprove, and to disprove a hypothesis it needs to be falsifiable (i.e. if the hypothesis is false, it must in principle be possible to show that it is wrong, and to do that there has to be some observation that would be impossible if the theory were true.). Note that mainstream climatologists do that all the time, that is exactly what climate models are for, making predictions that can be proven wrong by observation. Now where are the skeptics falsifiable predictions?

I pointed out this objection on Dr Spencer's blog, last time I looked (Saturday), he hadn't answered it and my subsequent posts are in some sort of moderation limbo, make of that what you will.

Actually, I think it is a little more subtle than a simple falsification question. If the 2nd-hand sources I'm reading are correct, Spencer has a hypothesis involving clouds, it's just not fleshed out to the same level of detail as the mainstream CO2 hypothesis.

So the ideal here would be to develop a detail climate model based on Spencer's hypothesis and test its performance against existing climate models. It might perform better or worse, although I'd expect several iterations of peer review before drawing a fair conclusion.

But here we run into a difference between 'the scientific method' and 'how science is done'. What I've described above is consistent with the scientific method (falsification, Bayes, etc), but falls down in practice because resources are finite. The system as described could be sabotaged by a flood of underdeveloped hypotheses which need to be tested and falsified. Indeed this may already be happening.

'How science is done' brings in extra factors - peer review, consensus. Things which have evolved as practical measures to make science work reasonably efficiently. Peer review as a junk filter, consensus as a mechanism to say 'stop - we've tested this hypothesis enough different ways, we haven't proven it (and never will), but it is good enough to be going on with'. Without these, science becomes bogged down (and thus Delingpole's peer-to-peer science can never work - under his model biology would do nothing but retest the theory of evolution over and over again to try and satisfy the IDers).

So, I think on the basis of 'how science is done', the onus is on Spencer to demonstrate that his model can provide predictions which are both testable and as specific, and at least comparable to those provided by existing models. If he can do that, the 'the scientific method' will come into play to test them further.

I may of course be giving him too much benefit of the doubt - after all I haven't read his papers and would not be qualified to judge them if I did.

Kevin C "Spencer has a hypothesis involving clouds, it's just not fleshed out to the same level of detail as the mainstream CO2 hypothesis."

That is essentially my point, Spencer has a hypothesis, but it is not yet a scientific hypothesis as it hasn't reached the point where it can make prohibitions on what we can observe so that it has the potenital of being falsified even if it is false. Until the null hypothesis is well-defined is meaningless to ask for papers where it has been rejected.

The funny thing is that climate modellers are perfectly happy to tell you what is consistent with climate variability, just look at the spread of the model runs (it is why model ensembles are useful). So we are in the funny situation where a challenge has been laid down where the challenger (apparently) won't say what is consistent with "natural variability" is, and won't accept the answers of climate modellers who will!

Seems like at least some of the "skeptics" are already abandoning the Global Cooling meme that they have been pushing for a number of years. I expected them to do that and replace it with "ok then, it is warming, but it is natural" sooner or later, but I had not expected it until 2012/13, as that will likely be the new record year.

Dikran: "Spencer has a hypothesis, but it is not yet a scientific hypothesis as it hasn't reached the point where it can make prohibitions on what we can observe so that it has the potenital of being falsified even if it is false. Until the null hypothesis is well-defined is meaningless to ask for papers where it has been rejected."

Oh yes, you're right. I hadn't picked out the full nuance there.

I think there is a rhetorical game going on too - labelling a particular hypothesis as 'null' in all but the most trivial cases has become a game of trying to grab the 'neutral' ground.

Re null hypothesis: In the absence of identifiable factors, the null hypothesis must be that the temperature is constant. As soon as there is some change observed, "natural" factors can not have any priority in explanatory models, this is plain natural science.

On the other hand, natural factors can of course not be dismissed before they are checked out of the case. But they can not be assigned any importance on the basis of speculation or belief, even if they should later turn out to have importance.

One basic problem with falsifying Spencer/Lindzen hypotheses about climate sensitivity, is that it is not a well-defined parameter that can be precisely estimated from a small set of observations. Rather, it is a "real" random variable, with a wide range of values possible - and observable. Therefore, it may be relatively easy for them to cherry-pick situations that seemingly support their assertions. As it is to pick examples supporting high sensitivity.

The relevant issue here is not singular realizations, but long-term averages under the conditions that will prevail during the next several decades. And here, observations tend to support common consensus. Which is not strange, that consensus is established from a wish to get the best possible estimates, whether high or low, not from hypotheses about low sensitivity that seem to disregard much of the available evidence.

Dana1981. From the article: Dr. Spencer has proposed an alternative to the anthropogenic global warming theory. He suggests that some unknown mechanism has caused global cloud cover to decrease over the past century.

Just wondering. If decreasing cloud cover lets more solar irradiation in, wouldn't ground based instrumental records show an increase in solar irradiation outside the solar cycle over the past 30 years? And since no increase over that period been shown, wouldn't that make Spencer's decreasing cloud cover hypothesis false?

It seems to me that characterising the issue as one of natural vs. anthropogenic causes is missing the point.

Carbon dioxide is a perfectly natural substance and its atmospheric concentration varies due to natural causes, so we only need to know whether and how that atmospheric concentration affects the global climate. In terms of the effect it has, it makes no difference at all whether a change in atmospheric concentration of CO2 is natural or anthropogenic. All we need to know is whether, all other things being equal, elevated atmospheric CO2 causes elevated global temperature, and by how much.

Similarly for methane, albedo, aerosols and so on - all are naturally occurring factors which affect global climate, and in terms of the physics involved it makes no difference at all whether they are changing naturally or due to human activities. The effect on climate will be the same either way.

So really the null hypothesis should be "atmospheric concentration of carbon dioxide has no effect on global climate" (and similarly for the other factors). Proving that these factors are changing because of human activities is a separate issue.

Spencer is really just setting up a false dichotomy between 'natural' and 'anthropogenic' causes, when the real issue is "does increasing CO2, CH4 & N2O cause global warming?" and similar questions.

He can't challenge AGW by citing imagined 'natural climate cycles', as if by discovering such cycles he could somehow change the laws of physics for the radiative properties of greenhouse gases. Science doesn't work that way.

If there was some unknown natural cooling factor offsetting the warming from the proven effects of increasing greenhouse gases then there would have to be yet *another* unknown natural factor having an equal and opposite *warming* effect to offset the unknown natural *cooling* effect, thus leaving the known enhanced greenhouse effect intact... which is the same as saying that the known enhanced greenhouse effect is what is causing the current warming in the first place.

SNR: "Re null hypothesis: In the absence of identifiable factors, the null hypothesis must be that the temperature is constant. As soon as there is some change observed, "natural" factors can not have any priority in explanatory models, this is plain natural science."

I'm not sure that's the case.

Suppose (fictitiously) that we knew from multiple measurements that the sun was getting warmer, and that this was universally accepted. However there was a debate over whether this was the only factor affecting the climate, or whether there were other environmental factors causing more or less warming than the solar forcing, then the null hypothesis would be warming from the sun alone.

So I think that the idea of a null hypothesis is actually often a social construct. In the type of simple cases you get in statistics and philosophy texts there is often a hypothesis that uncontroversially fills the 'null hypothesis' space. But I'm not sure that that is generally the case.

But maybe the null hypothesis is determined by the question being asked? So if we ask 'is there warming', then the null will be no warming. If we ask, for our fictitious planet, 'is there non-solar warming', then the null will be solar-explained warming.

But what happens if we reword the question? How about if we ask 'is there no warming?'. OK, that's an obvious trick . But if we have a different set of preconceptions from which the hypothesis of no warming looks surprising, then that becomes a sensible question. So I think I come back to the position that in non-trivial cases null hypotheses are socially constructed. In the case of a consensus, then the null hypothesis is primarily shaped by the consensus position.

If that's correct, then the null hypothesis among scientists would be a climate sensitivity of ~3C, and the onus is again on Spencer to provide an alternative hypothesis. Of course we also exist in a social and media environment in which the consensus may be different. Which means you have to shift your null when talking to different groups. Messy!

A different way to state many of the comments here might be the following:
For over 100 years there has been a developing theory that anthropogenic CO2 would warm the earth's climate. Then, in the past 30 or so years, evidence has been building that the climate is warming. The scientific process is then obligated to test the most prevalent theory, and it seems to me that is what has been happening in climate change literature. The theory of AGW via CO2 emissions consistently rises to meet all challenges, to date, while other hypotheses are either not sufficiently flushed out to have become "theories", or they are "flushed out" by the process.

Spencer's complaint seems to miss these points entirely. He has every right to look for an alternative mechanism, but until he or someone else comes up with something that can be tested, it does not merit the time of the scientific community to test. It seems to me that there are some in the climate modeling community that could do a great service by taking a shot at parameterizing models with values provided by Spencer. If he cannot provide the parameters, then he has little to complain about.

Dr. Spencer's challenge has validity. I do think it could have been worded better. It should be noted that his challenge does allow for the influence of anthropogenic CO2 on the current warming.

What we know:
1. Climate has changed in the past due to external and internal forces.

2. Even with the higher CO2 levels, this interglacial period appears to have maxed out at more than 2 degrees on average below the previous 4 interglacials.

3. CO2 is a greenhouse gas and helps contribute to the vital warming of the planet.

4. Through the combustion of fossil fuels and landuse practices - man has contributed CO2 to the atmosphere and is partially responsible for the current rise in CO2 levels.

Dr. Spencer is asking for a paper ruling out natural, internal climate cycles as the cause of "most" of the recent warming. That certainly leaves the door open for the contribution of CO2, and more specifically anthropogenic CO2, to the mix.

According to one of your links above, in the past 150 years 50% of the 0.8 degree C rise in surface temperatures can be attributed to man. This isn't the place to squabble over those figures, but for the sake of discussion that is a contribution, not a cause.

Most of us "deniers" (we could just as easily create a label called natural climate change deniers), agree that CO2 contributes to warming which contributes to climate change. We also think that man's land use practices contribute to weather and climate. However, it takes a leap of faith to abandon the repetitive, observed natural climate changes over the history of the Earth (both on the short and long scales) in favor of climate change wholly induced by the actions of humanity.

Icarus wrote: "All we need to know is whether, all other things being equal, elevated atmospheric CO2 causes elevated global temperature, and by how much."

The fact that the rise in atmospheric CO2 is rising due to anthropogenic emissions is easily established*, hence if we know elevated atmospheric CO2 causes global temperatures to rise and by how much, we automatically know it is due to our activity.

"So really the null hypothesis should be "atmospheric concentration of carbon dioxide has no effect on global climate""

Spencer can't use that as the null hypothesis as he says the warming (without feedback) from a doubling of CO2 is about 1 degree F, and that that statement is not contentious (see here). Very few skeptics think that CO2 is not a greenhouse gas, so they can't use a null hypothesis that they know from basic physics must be false. They generally argue that climate sensitivity is low and that negative feedback will prevent much warming (one wonders what happened to the negative feedback during the PETM event)

Dr Spencer's null hypothesis is the right one, just inadequately specified; however the only way to specify what is plausible under "natural variability" is to build a model incorporating the known physics of the natural climate (given observed forcings other than anthropogenic emissions), and that has already been done and published in the IPCC WG1 scientific basis report.

*If the natural environment were a net source of CO2 to the atmosphere, then (assuming conservation of mass) the annual rise in atmospheric CO2 would be greater than anthropogenic emissions, however the observations show this is not the case, the rise is only about 45% of anthropogenic emissions, so we know the natural environment is a strong net carbon sink, and hence is not the cause.

@apiratelooksat50 For a hypothesis to be a valid scientific theory it needs to be falsifiable. If you think the hypothesis is valid, give me an example of an observation that would falsify it (i.e. an observation that would be impossible if the hypothesis were true).

I would enjoy CSI: RSVP where corpses lie around with bullet wounds and axes sticking out of them and they are all deemed to have died of 'natural causes'.

00

Moderator Response: [Dikran Marsupial] ;o) However, can we keep comments substantive (I am thinking here of RSVP's orignial post rather than your reply- the use of "natural" to mean "non-anthropogenic" is well established and causes no genuine misunderstanding).

A couple of points are worth noting. First, as discussed in the article and in more detail by Mark in #8, Spencer does sort of have an alternative hypothesis. It's that some unknown mechanism is causing cloud cover to change, perhaps through some unknown heat redistribution. It's not exactly just hand-waving "natural cycles", though I don't think it's far from it. I agree with Mark that it's the most plausible alternative to AGW put forth, but that's not saying much.

Secondly, scientists can choose any null hypothesis they want for any given study. If they want to disprove "natural" causes, they can set the null hypothesis to "global warming is being caused by natural effects". Spencer's problem is that almost all climate scientists are convinced the warming is anthropogenic, so that's effectively their null hypothesis. Basically he wants them to continue trying to disprove "natural causes" when they're already convinced it's anthropogenic.

My reaction to that is "tough sh*t". Like I said, any study can set any null hypothesis it wants. Spencer is free to attempt to disprove AGW, or attempt to prove his "internal radiative forcing" hypothesis. But the evidence is overwhelmingly on the side of AGW, so climate scientists are entirely justified in using it as their null hypothesis.

apiratelooksat50 #25 - 50% is the minimum amount of warming over the past century that humans can be resposible for, unless climate sensitivity is below the consensus range of many different lines of evidence (2 to 4.5°C for 2xCO2). And physics is not a leap of faith.

Moreover, I don't think there have been any cases of 0.8°C warming in 100 years or 0.55°C warming in 35 years previously during the Holocene. Thus there is a difference between current and previous warming events.

Not being able to disprove the null hypothesis does not disprove the alternative hypothesis. Nobody can prove that evolution didn't happen purely by random mutation rather than being driven by natural selection, but that doesn't mean that the theory of Darwinian evolution "necessarily fails".

(we could just as easily create a label called natural climate change deniers),

Not without being hugely dishonest, you couldn't. The current understanding of AGW is partly the result of a long process of ruling out known mechanisms for natural climate change. An unwillingness to take vague, unsubstantiated guesswork about "unknown mechanisms" seriously is not "denial"; it's a demand for evidence. The fact that you can't provide that evidence is not our fault, or our problem. It's yours.

it takes a leap of faith to abandon the repetitive, observed natural climate changes over the history of the Earth (both on the short and long scales) in favor of climate change wholly induced by the actions of humanity.

No, it takes decades of observations and careful data-gathering by people who know much, much, much more about the science than you do. The fact that you refuse to accept those observations and that data because they make you ideologically uncomfortable is not our fault, or our problem. It's yours.

I pointed out when his challenge hit WUWT that it's somewhat underhanded for Spencer to demand only peer-reviewed literature to use against him when he refuses to even attempt publishing in peer-reviewed venues anymore. Refereed letters journals like Geophysical Research Letters (the only outlet he chooses besides his own website) are not sufficient by themselves as a platform to credibly fight off full-fledged papers in Science or Nature. I also mentioned that Spencer himself has obligations to live up to in establishing the validity of his own "null hypothesis" if he wants to play the game.

Conversely, if one generates an hypothesis that is impossible to disprove because it is all encompassing that doesn’t mean that the hypothesis is true. If I propose that leprechauns are causing increased snowfall, decreased snowfall, increased drought, increased floods, higher temperatures, colder temperatures, melting sea ice, increased number of hurricanes (but only in some places), tree ring divergences (only in the north), increased species extinctions (everywhere), rising sea levels, changes in cloud cover (increased and decreased), etc. then it would be hard to prove me wrong.

you have to admit that there should be some research showing how natural forces are at least coupling with AGW to produce the warming. Are we afraid to ask the question?

Gary @ 39
I think we agree with each other. The natural forces didn't just go away because humans and fossil fuel combustion showed up. And, CO2 whether from humans or the Earth contributes to the greenhouse effect.

apiratelooksat50 - "Are you stating those forcings are not factors now?" Strawman argument!

Please read the topic and discussion on CO2 is not the only driver of climate. CO2 accounts for most of the temperature change of the last 50 years, but not all. Nobody has claimed otherwise.

Again, the question is one of evidence. There's plenty of evidence that our CO2 emissions are changing the situation, and no evidence that undefined "natural cycles" are doing so. The well defined and understood natural forcings and cycles cannot account for recent climate behavior - natural forcings plus our greenhouse gases do.

Claiming these undefined, untestable cycles are responsible, rather than well established physics, is the equivalent of claiming it's leprechauns or (as I have phrased it), the lawn gnome Illuminati. It's quite irresponsible.

This has been looked at before, in great detail, by very sharp and knowledgable minds:

Only the anthropogenic contribution completes the picture, explaining the warming we can empirically see and measure in the absence of other forcings. Else we would be measuring a decades-long cooling trend. Which we aren't.

Regarding Spencer's challenge. It is iMHO very underhanded of him and probably an effort to fabricate "debate". It also just reinforces pre-conceived notions by some that the mainstream climate science ignores natural variability (which is most definitely does not, start reading the IPCC report here) or that the warming can me attributed to some mystical, hitherto undiscovered driver. The scientific literature abounds with example of papers investigating the role of natural variability on regional and global temperatures and precipitation.

Spencer's "challenge" really does smack of desperation and wishful thinking.

Regarding clouds, that is a tricky one. As Norris (an expert in the field) data issues are a limiting factor in elucidating the role of clouds (see Norris and Slingo (2009). That said he has tried, very hard I might add, to identify trends in low clouds and their possible contribution to the observed temperature changes. His recent paper in Science (Clement et al. 2010) finds evidence for a positive feedback, with warming leading to decreased low-level cloud cover which in turn causes more warming.

In a presentation made by Norris in 2009, he made the following statements after considering all the best available data:

And from Norris' "summary":
• cloud changes since 1952 have had a net cooling effect on the Earth

He also of the opinion that:
"internal climate variability is unlikely to produce cloud trends occurring over the length of 46 years
• anthropogenic or natural external forcing is unlikely to directly produce the observed cloud trends
• an internal response of the climate system to external forcing could produce the cloud trends
• there is not yet enough information to attribute the cloud trends to anthropogenic global warming"

"For example, these questions remain to be answered with complete confidence:
•How much warming has occurred due to anthropogenic increases in atmospheric trace-gas levels?
•How much warming will occur in the future?
•What other changes will occur with future warming?"

To have some common ground where we can discuss what is happening to the climate, this gives us a good basis that I think we can both work with.

Also, has the data used to generate that graph been used elswhere? It doesn't look to scale, especially if we've only had 0.8 degree C warming in the past 150 years.

garythompson #39 - as other comments have noted, climate scientists have examined the coupling of natural forcings with anthropogenic ones. My favorite example is Meehl et al. (2004), which I believe is the source of Daniel Bailey's figure in comment #44.

Albatross #46 - thanks! I agree, Spencer's challenge is implying that other climate scientists ignore natural forcings, which is clearly not the case. Good reference to Norris too.

KR @ 43
I am not misrepresenting your argument. I'm asking a clarification so I do not do just that.

The Earth's climate has regularly cycled through glacial and interglacial periods. There has to be a force or forces to cause that. Those changes are observable. Milankovitch cycles have been proposed as one of the possible forces. Through other postings on this site it is evident that CO2 lags temperature, though they do appear linked otherwise.

I'm on record of saying that I am willing to consider CO2 as a contributor to global warming.

Dana,
Thank you for this useful summary. However, there is one statement that bothers me:

"Another human fingerprint is the higher rate of warming at night than during the day." This statement is presented as if it is obvious. It's not. Can you explain why "greenhouse gases are able to make more of a difference in the surface temperature" at night?

Braganza 2004 and Alexander 2006 do show that nighttime warming has been greater than daytime warming. Braganza also shows the change in diurnal temperature range (DTR) as predicted by GCM simulations. The predicted change is negative, but is almost consistent with no change. The observed change in DTR is also negative, but it is approximately 3 times as large as the predicted change. With that sort of discrepancy between the data and the model, one can't credibly claim that the data support the model.

00

Moderator Response: [Daniel Bailey] This prediction goes all the way back to Arrhenius in 1896, was confirmed by (among others) Dai et al 1999, Sherwood et al 2005 and is discussed in detail here.